This invention relates to a model-based upshift control for an automatic transmission, and more particularly to a control that manages both engine torque and on-coming clutch pressure.
In general, a motor vehicle automatic transmission includes a number of gear elements and selectively engageable friction elements (referred to herein as clutches) that are controlled to establish one of several forward speed ratios between the transmission input and output shafts. The input shaft is coupled to the vehicle engine through a fluid coupling such as a torque converter, and the output shaft is coupled to the vehicle drive wheels through a differential gearset. Shifting from a currently established speed ratio to new speed ratio involves, in most cases, disengaging a clutch (off-going clutch) associated with the current speed ratio and engaging a clutch (on-coming clutch) associated with the new speed ratio.
It is well known that transmission shifting can be controlled by manipulating the clutch pressures and the engine torque output during the shift for improved shift quality and transmission durability. See, for example, the Lockhart et al. U.S. Pat. No. 4,724,723, assigned to the assignee of the present invention, and Nitz et al. U.S. Pat. No. 5,129,286, assigned to Saturn Corporation. Nitz et al. use open-loop engine output torque controls to suppress engine flare during low torque upshifting, while Lockhart et al. employ closed-loop engine torque and on-coming clutch pressure controls to maintain a desired constant output torque. Other open-loop and closed-loop clutch pressure controls are respectively described in the Downs et al. U.S. Pat. No. 4,707,789 and Hibner et al. U.S. Pat No. 5,058,460, both of which are assigned to the assignee of the present invention.
The present invention is directed to an improved control for an automatic transmission upshift, wherein the engine output torque and on-coming clutch pressure are coordinated during the shift based on an inverse dynamic model of the transmission to achieve a desired output torque trajectory. The desired output torque trajectory is influenced by operator demand, and an initial value of the desired output torque trajectory is used along with the engine output torque to develop an input acceleration trajectory. The inverse dynamic model of the transmission is used (1) to determine an engine torque command that will achieve both the input acceleration trajectory and the desired output torque trajectory, and (2) to determine a feed-forward pressure command for the on-coming clutch that will produce the input acceleration trajectory, given the engine torque command. The desired output torque trajectory is used to determine the expected input speed, and a feed-back control term based on the deviation of the measured input speed from the expected input speed is used to adjust the on-coming clutch pressure command to account for model errors. Finally, the closed-loop feedback error is used to enable adaptive correction of the feed-forward control so that the feed-forward clutch pressure more nearly produces the commanded input shaft acceleration. Coordinating the on-coming pressure and engine output torque in this manner achieves more consistent shift feel and energy dissipation in the on-coming clutch, with less intensive calibration effort and improved adaptability to different powertrain an vehicle-type configurations.